A field-effect transistor may include a gallium nitride (GaN) electron channel layer using an aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterojunction. GaN is a material having a relatively wide band gap, high breakdown voltage, and high saturated electron velocity. Hence, GaN is a promising material for forming a semiconductor device that implements high current, high withstand voltage, and low on-resistance operation. Therefore, numerous studies are being conducted on the development of semiconductor devices having GaN based materials including the field-effect transistor having a GaN electron channel layer for next generation high-efficient switching devices that may be able to save power in an amount exceeding the limits of silicon power devices.
In semiconductor devices such as the field-effect transistor, an insulating film may generally be formed on an entire surface of the field-effect transistor or the like due to passivation or the like after a gate electrode or a drain electrode has been formed.